Previous vacuum waveguide flange systems designed to align and connect microwave components and to carry high peak power RF energy have all had one thing in common: they use some form of knife edge in combination with some form of a crushable gasket to achieve both a vacuum and an RF seal with the same structure. One such flange (the “SLAC flange”) was designed at the Stanford Linear Accelerator Center (“SLAC”) and has been in worldwide use for decades. See, e.g., W. R. Fowkes, et al., “High Power RF Window and Waveguide Component Development and Testing above 100 MW at X-Band”, SLAC-PUB-5877, August, 1992. FIG. 1 is a cross-sectional view of this prior art flange device having mating flanges with a copper gasket and integrated clamping mechanism. Two thick stainless steel flanges 10 (male) and 12 (female) are bolted together with a copper gasket 14 in between them to align and connect two waveguides 16 and 18. The flanges 10, 12 are designed to mate together in a plug and socket (or male and female) configuration. The plug and socket fit together tightly to provide good alignment of the waveguides. They also contain the knife edges 20, 22 that, when compressed into the gasket 14 under the action of the flange clamping hardware (bolts, nuts, washers) 24, create the RF and vacuum seals. The SLAC flange knife edges 20, 22 are angled and precisely rounded with a small radius. They are offset from one another on the mating flanges. This helps to compress and form the gasket in a large area onto both the angled and rounded surfaces of the knife edge to ensure the vacuum seal. Compression is typically 0.008″ per side and is limited by a hard flange stop 26 on the flange. Compression also causes the gasket to move inward towards the interior 28 of the waveguides 16, 18. The gasket 14 is made to be slightly larger than the waveguide opening so that when it moves inward under compression it is flush or nearly flush with the waveguide surfaces. The interior 28 of waveguides 16, 18 is under vacuum in operation and is where the RF power signal 30 is carried. A more recent sexless embodiment of the flange system has been developed by the European Organization for Nuclear Research (“CERN”). See, e.g., P. Lutkiewicz, et al., “Design of a New UHV All-Metal Joint for CLIC”, Vacuum, Vol. 84, pp. 289-292, 2010.
An advantage of such a flange is that it has been designed to be quickly and easily disassembled and reused. Disassembly is accomplished by loosening and removing the flange clamping hardware 24, pulling the two mating flanges 10, 12 apart and removing the now-crushed gasket 14. The flanges 10, 12 can then be reconnected by inserting a new gasket 14 and using new flange clamping hardware 24. A disadvantage of this prior art flange system is the possibility of developing vacuum leaks over time if it is subjected to thermal cycles. This could be due to heating during use (such as by passing high average RF power through it) or by high temperature processing which is commonly done to improve the vacuum conditions in systems that use such flanges. The knife edge is similar to that found on commercially available vacuum flanges, such as the “Conflat flange”, that have been in use for decades. See U.S. Pat. No. 3,208,758 entitled “Metal Vacuum Joint” by Maurice A. Carlson and William R. Wheeler (Varian Associates), 1965. The manufacturer does not recommend subjecting this flange to temperatures above 400 degrees C. because of its tendency to develop vacuum leaks due to the differential expansion forces between the bolts, flange and gasket which can result in the knife edge being pulled away from the gasket. The same phenomenon occurs in the SLAC and CERN flanges and other flanges using the same principle of achieving the vacuum seal. Once a vacuum leak occurs, recompression of the gasket is usually not effective at resealing the flange for a number of reasons but mainly due to the fact that the hard flange stop 26 limits or prevents altogether any further compression.
A further disadvantage of these flanges is the need to be of a certain size to function properly. The outer dimension is controlled by the size of the waveguide to be connected, the minimum number of bolts needed to uniformly and adequately compress the gasket to achieve the vacuum seal and the minimum separation between bolts to allow for the use of tools to tighten them. The flange must also be relatively thick to withstand without distortion the large compressive force from the bolts during assembly. This results in an electrically long length for the flange interior surfaces 32, 34. These flanges must be made of a strong material which is typically stainless steel. The moderately high electrical resistivity of this material attenuates the RF power signal 30 that is being propagated in the waveguide yielding increased power losses and local heating. Propagation through a number of these connections would result in a noticeable loss of signal. Typically, these losses are greatly reduced by the application of copper plating, which has a much lower resistivity, to the flange interior surfaces 32, 34. This is usually a two-step process where the entire flange is plated first then all of the plating is stripped off except on the interior surfaces. This adds some cost to the manufacturing process and may increase the likelihood of arcing during use if the plating exhibits nodules, pitting or adherence issues.